The retinoblastoma protein (Rb) is critical for suppressing cancer, regulating cell proliferation, and inhibiting cell death in the retina, retinal pigment epithelium, lens and other ocular tissues. The long-term goal of our research is to understand the importance of Rb in ocular health and disease, which may allow us to develop new therapies for eye disorders. Rb localizes to gene promoters through its interaction with E2F transcription factors, and it regulates gene expression by assembling multimeric chromatin remodeling complexes. Rb itself is regulated by two key phosphorylation events: phosphorylation of the C-terminus blocks the ability of Rb to inhibit cell proliferation, whereas additional phosphorylation of Rb at serine-567 blocks the ability of Rb to inhibit cell death. Concurrently, we have shown that Rb is critical for the differentiation and survival of ocular melanocytes - a type of pigment cell that plays an important role inthe pathogenesis of ocular melanoma, albinism, microphthalmia and other eye diseases. When Rb is minimally phosphorylated, it is in its most active form and is able to cooperate with the microphthalmia transcription factor (MITF) to induce melanocytes to differentiate and cease proliferating. When Rb becomes partially phosphorylated, it looses the ability to inhibit cell proliferation but it still blocks cell death. When Rb is completely inactivated by phosphorylation of serine-567, it can no longer prevent cell death. Appropriately, serine-567 phosphorylation occurs only in abnormal cells. We hypothesize that the separate regulation of cell proliferation and cell death by Rb according to its phosphorylation state serves as a buffer against inadvertent cell death during normal cell proliferation while providing a mechanism for eliminating abnormal cells that could lead to cancer and other diseases. Using our ocular melanocyte model, we propose a series of experiments organized around three specific aims to determine how Rb regulates cell proliferation, differentiation and cell death in ocular melanocytes. Understanding how Rb accomplishes these functions could result in new treatments to eliminate cancer cells and alternatively, to prevent the loss of normal cells and encourage tissue regeneration in eye diseases such as macular degeneration and retinitis pigmentosa. Consequently, these aims are highly relevant to the vision statement of the NEI, and they address several major program goals and objectives of the Retinal Diseases Program.